Grid-connected distributed power generators such as photovoltaic (PV) systems typically consist of two major parts: PV cell arrays to convert irradiation to electrical energy, and an inverter to feed the electrical energy into a power distribution grid. PV cell configurations may be categorized according to four groups [1]: centralized, string, multi-string, and AC-module and AC-cell technologies, see FIG. 1(a). All approaches have advantages and disadvantages [1], [2], and compromise various attributes such as harmonic rejection capability, simplicity, efficiency, flexibility, reliability, safety, modularity, and cost.
The centralized topology is considered the best for high power applications because the input power level can be increased, and it uses only one inverter which increases the conversion efficiency. However, this topology has limitations. For example, since there is significant high voltage DC wiring between the PV cells and the inverter, the topology requires expensive DC switches and special isolation, safety, and protection circuits. Due to the centralized maximum power point tracking (MPPT), partial shading of the PV cells or any mismatch between the PV cells causes a significant drop in the output power generation.
For medium power applications, the most suitable configuration is considered to be the string or multi-string technologies, [3], where one or more strings of PV cells are connected to a single inverter, as shown in FIG. 1(b). Unlike the centralized configuration, this type of configuration enables independent MPPT for all strings which might be installed in different sizes and orientations. This also increases the overall efficiency under certain circumstances, such as partial shadowing of the PV cells. Therefore, this topology offers the flexibility to optimize the number of strings and inverters for the specific application power level to increase the overall efficiency and to reduce losses.
Since the PV cell array current-voltage characteristic is highly nonlinear, the MPPT of PV cell arrays is challenging. MPPT systems usually consist of two parts; MPP tracker hardware, and an algorithm. The MPP tracker alters the input resistance of the inverter seen from the output terminal of the PV cells that results in a change of the operating point. MPPT algorithms [4] calculate the best operating point available based on the current irradiation and temperature of the PV cells and provide a reference point for the MPP tracker hardware.
In single-phase or unbalanced three phase grid-connected systems, the instantaneous power injected to the grid oscillates at twice the grid frequency. One of the MPP tracker tasks is to decouple the power oscillation from the PV cells, because the oscillation results in a deviation from the optimum operating point of the PV cells [2], [5]. This problem is usually resolved by connecting a large electrolytic capacitor at the PV cell terminals, which in turn decreases the lifetime and increases the volume, weight, and cost of the inverter. To avoid the electrolytic capacitor, an auxiliary circuit may be used [6], [7] which draws constant current from the input and generates a high DC voltage at the middle stage to supply the pulsation required at the output. In [8], an auxiliary circuit was proposed with a transformer and passive and active components to avoid oscillation. However, such solutions have low efficiency and have complex hardware and control systems, which make the overall system expensive.